1. Field of the Invention
This invention relates generally to corpuscular or charged-particle beam microscopes, such as electron microscopes, and in particular to corpuscular beam microscopes which include means for determining and correcting drift of the object image.
2. Description of the Prior Act
Corpuscular beam microscopes of the foregoing type are known in the art. For example, applicant's co-pending patent application Ser. No. 602,461 filed Aug. 6, 1975, now U.S. Pat. No. 3,971,936, discloses a corpuscular beam microscope which includes means for adjusting the position of the object to be imaged relative to the equipment axis of the microscope and/or relative to the focusing plane of the objective lens of the microscope on the object side and/or for changing the position of the object image relative to the equipment axis of the microscope. The microscope also includes means for controlling the condition or state of the object image by means of a control signal which is derived from a convolution of the actual object image with a stored reference object image. The control signal is utilized to control the adjusting means of the microscope and correct image drift in the microscope, and comprises a correlation signal which is derived from the instantaneous value of the correlation integral of an actual object image (the instantaneous microscope image) with a reference object image (a microscope image at an earlier instant in time).
The basic idea of the invention disclosed in the foregoing application is to correlate the instantaneous object image, known as the "actual object image", with a previous state of itself, known as the "reference (or desired) object image". The convolution operation is performed in the microscope with the reference object image rotated 180.degree. relative to the actual object image about a normal to the plane of the image. Mathematically, this convolution operation is carried out by calculating the integral P.sub.u,v with P.sub.u,v = .intg..intg. f(x,y)f' (x+u,y+ v)dxdy, where f represents the actual object image, f' represents the reference object image, and u,v represents the line segments in the direction of the coordinate axes by which the reference object image is displaced relative to the actual object image. The integral P.sub.u,v is known as the "correlation integral". This integral has its maximum value when there is substantial coincidence between the actual and the reference object images, i.e., when the reference object image is in approximate registration with the actual object image. This maximum value of the correlation integral is known as the correlation maximum.
In one embodiment of the invention disclosed in applicant's foregoing co-pending application, a control variable or signal is utilized to control the positioning devices of the microscope which is derived from the correlation maximum. The correlation maximum is formed in this case by a light-optical device at a point of a planar detector. The reference object image, which is realized as a light relay, e.g., a photographic plate, is illuminated by the acutal object image. The actual object image, the reference object image, and the detector of the microscope are disposed along an optical axis at predetermined, fixed distances from each other, and the planes of the two images and the detector are perpendicular to this axis.
If the origins of the reference and actual object images lie on the optical axis, then the correlation maximum will be located at the point where the optical axis passes through the surface of the detector. If the actual object image is displaced perpendicular to the optical axis, however, the correlation maximum will lie outside the optical axis. The location of the correlation maximum on the planar detector is, thus, an indication of the displacement of the actual object image relative to the reference object image.